Traditionally (in the days before readily available mass-produced patches), a shooter who had just fired would blow down the muzzle to clear the vent to the pan and to let humidity from his breath soften the black powder fouling and hopefully put out any ember that might remain in the barrel. Then place a large piece of cloth over the barrel, short start the roundball until just below flush with the muzzle, then use a knife to cut off the rest of the cloth. Since the gun (which had an empty pan and didn't even have a removable breech plug, much less a second shot) had just been discharged, there was nothing unsafe about it.
However, the "Ten Commandments of Gun Safety" dictate that you always treat every gun as though it is loaded and never point any gun at something you don't want to shoot. That would include one's head.
I have no real problem with those who have long practiced the habit. However, modern primers will much more forcefully clear the nipple/vent than your breath will, and there should hopefully be less fouling. Additionally, black powder is no longer home- or locally-made and hand-ground from charcoal made from chunks of wood from a dogwood or ash tree, so it should be much more uniform and small-grained--resulting in much less chance of a stray ember remaining in the barrel.
Most importantly, the best way to clear fouling (or to make sure all embers are extinguished) is to spitpatch between shots. It's much more likely to put out embers and it wipes away the fouling, not just moistens it. Personally, I can't imagine why anyone would NOT spitpatch between shots. Accuracy is built on uniformity and consistency, and spitpatching (a spitpatch followed by a dry patch) between shots will keep your barrel uniformly fouled, instead of the barrel becoming progressively fouled the more you shoot.
